A. Field of the Invention
The present invention relates to digital imaging, and in particular to control systems for processing digitally stored images prior to output to imaging devices.
B. Description of the Related Art
Various methods and technologies exist for encoding images digitally and transferring the digital representations to output devices. At the encoding stage, these range from hobbyist scanners and graphic-arts software to elaborate prepress systems, and at the output stage can include laser printing, digital exposure of photographic films, and transfer of the image to lithographic plates for subsequent mass-quantity printing. In the latter case, suitable techniques of plate production include the use of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, to create physical or chemical changes at selected points of sensitized plate blanks, which are used (immediately or after exposure to conventional development processes) for planographic printing; ink-jet equipment used to selectively deposit ink-repellent or ink-accepting spots on plate blanks, also to produce planographic printing plates; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to alter the characteristics of certain areas on a printing surface, thereby producing "dots" which collectively form a desired image. As used herein, the term "imaging device" includes radiation sources, ink-jet sources, electrodes and other known means of producing image spots on blank printing plates, and the term "discharge" means the image-forming emissions produced by these devices.
In a typical laser-based imaging configuration, laser output is provided directly to the surface of a substrate via lenses or other beam-guiding components, or transmitted to the surface from a remotely sited laser using a fiber-optic cable. A controller and associated positioning hardware maintains the beam output at a precise orientation with respect to the substrate surface, scans the output over the surface, and activates the laser at positions adjacent selected points or areas of the substrate. The controller responds to incoming image signals corresponding to the original document or picture being copied onto the substrate to produce a precise negative or positive image of that original. The image signals are stored as a bitmap data file on a computer. Such files may be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can accept input data in page-description language, which defines all of the features required to be transferred onto the substrate, or as a combination of page-description language and one or more image data files. The bitmaps are constructed to define the hue of the color as well as screen frequencies and angles.
The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the substrate mounted to the interior or exterior cylindrical surface of the drum. In the case of lithographic printing, the exterior drum design is more appropriate to use in situ, on a lithographic press, in which case the print cylinder itself constitutes the drum component of the recorder or plotter.
In the drum configuration, the requisite relative motion between the laser beam and the substrate is achieved by rotating the drum (and the substrate secured thereon) about its axis and moving the beam parallel to the rotation axis, thereby scanning the substrate circumferentially so the image "grows" in the axial direction. Alternatively, the beam can move parallel to the drum axis and, after each pass across the substrate, increment angularly so that the image on the substrate "grows" circumferentially. In both cases, after a complete scan by the beam, an image corresponding (positively or negatively) to the original document or picture will have been applied to the surface of the substrate.
Multiple imaging devices may be used to produce several lines of image spots simultaneously, with a corresponding increase in imaging speed. Regardless of the number of imaging devices used, their operation must be precisely controlled so that the discharges occur at the appropriate times to reach the intended dot locations on the printing surface. Each discharge source must be aligned with the substrate along longitudinal and lateral dimensions (corresponding to circumferential and axial directions in the case of drum imaging) at all points during a scan of the all candidate image points on the substrate, and, in the case of laser-based imaging, the beam must remain focused on the substrate for maximum energy-transfer efficiency.
Failure to maintain proper alignment along all relevant dimensions results in imaging inaccuracies and/or undesirable periodic artifacts that detract from the final image appearance. The consequences can be particularly acute in planographic printing contexts, since typical print jobs require sequential application of ink from several plates, each of which is vulnerable to image degradation if created digitally; the result is a cumulative aggregation of the imperfections associated with each plate. Laser imaging imposes perhaps the most demanding requirements, since adjustments along each of the three dimensions can result in introduction of distortions along the other dimensions.
Especially troublesome printing artifacts result from the use of multiple imaging devices. One class of artifact, described in U.S. Pat. No. 5,182,990 (the entire disclosure of which is hereby incorporated by reference), occurs in multiple-device discharge-type imaging systems when, for example, individual devices fail to image at the same intensity as other devices, the writing head is improperly oriented, or individual imaging devices within the head are improperly aligned. The solution taught in that patent calls for staggering with respect to one another the initial imaging positions of the writing heads assigned each plate. This strategy prevents artifacts caused by similarly situated elements within the heads from appearing at the same positions on the plates, and thereby reinforcing one another in the final printed image.
Another class of periodic artifact, not amenable to solution according to the techniques described in the '990 patent, occurs primarily with use of laser or other beam-type imaging devices as a result of imperfect registration between regions imaged by different devices. Misregistration produces uneven "seams" between the regions occurring along the direction of imaging, and which tend to be visible to the eye even when imaging takes place at high resolutions.